Backlights of display devices will gradually decrease in luminance due to aging and deterioration. For this reason, display devices suffer from a problem that backlight luminance becomes lower than a setting value a half year or one year later than the timing at which backlight luminance was adjusted to a desired setting value.
To solve the problem, for example, it is possible to introduce a method in which a luminance sensor is used to measure the luminance of a backlight so as to correct the backlight luminance such that its measured value will match the setting value. However, this method needs to use a luminance sensor with a relatively high cost. In addition, the luminance sensor including precision optical parts may easily cause an erroneous operation due to dirt, discoloration, and deformation.